Can a imperial use psychic powers from different discipline or he absolutly have to select from a single discipline?

The various Psy Rating talents will tell you what you can pick. Usually you will get the option of a number of powers from a discipline or the option to get a power from a new discipline.

^^^^^ Exactly

For example:

The first level you actually get to select a from a major psychic discipline is level 4. You must pay experience for Psy Rating 3. Once you have this you can select any major psychic power from that 1 discipline you chose. So say you wanted to take the major PP Seal Wounds from the discipline Biomancy you would first have to declare that Biomancy is your discipline you are going to choose for Psy Rating 3. Later, if you then want to pay for another PP you are still limited to only select a major PP from Biomancy. You CAN'T select Holocaust until you increased your Psy Rating to 4 and gain knowledge of Pryomancy or another discipline.

Does that makes sense? Hope it helps.